US immigration officials are racing against time to comb through years of data in a search for up to 10 trained al-Qaida hijackers who, investigators believe, are still on the loose in America.

Evidence has emerged on both sides of the Atlantic that indicates there were plans to hijack two more airliners on September 11 and that several of the would-be hijackers have yet to be caught.

Since September 11 there has been speculation over the existence of more hijack cells, apart from the 19 terrorists who took over four airliners and crashed them into the World Trade Centre, the Pentagon, and a rural area in Pennsylvania.

Following the discovery of fresh documentary evidence, investigators in the US and Europe are working on the assumption that 30 terrorists, in six teams, were intended to have taken part in the onslaught. It is thought at least one plane was to have been aimed at the White House.

An intelligence source in the US said another plane due to have been hijacked was a Continental Airlines flight from Newark on the morning of September 11. Retractable knives (similar to Stanley knives) of the same type used in the four successful hijackings were found taped to the backs of fold-down trays.

The source did not give details of the sixth plane. However, similar knives were found stashed in the seats on a plane which had been due to leave Logan airport in Boston the same morning, and which was delayed and then cancelled.

The new evidence specified the number of hijackers involved but did not provide all their names, an intelligence source in the US said. Two suspected members of the Hamburg-based cell where the plot is thought to have been hatched are missing.

Both men, Said Bahaji and Ramzi bin al-Shibh, are thought to have formed part of the operation's back-up structure and are thought to have fled to Pakistan.

The only named suspect thought to have been part of the September 11 team is a Moroccan-French man in custody in New York. Zacarias Moussaoui is seen as a possible fifth member of the hijack team which seized United Airlines flight 93, the plane that crashed in Pennsylvania.

Moussaoui, who lived in London before leaving for the US early this year, was detained on August 17 after his requests for flying lessons on an airliner simulator aroused suspicion. He possessed only a student pilot's licence and wanted to learn how to turn and fly straight, not how to take off or land.

Two Indian citizens, Mohamed Azmath and Ayub Khan, have also been under suspicion of involvement. They were arrested on September 12 in Texas on a train from St Louis to San Antonio.

On the morning of September 11, they had boarded a flight from Newark to San Antonio, but it had been diverted to St Louis after news came of the first impact in New York. The two men were found with retractable knives, thousands of dollars, and hair dye among their possessions.

There have been reports that both held a commercial pilot's licence in India, although before September 11 they had low-paid jobs at a newspaper kiosk. Somehow they could afford to send $64,000 (£45,000) in wire transfers to India in 1999.

However if the men, both from Hyderabad, were would-be hijackers it is not clear why they did not seize their plane. Also, they were on a TWA flight from Newark and knives were reportedly found hidden on a Continental airliner.

Neither man is cooperating with investigators, nor is Moussaoui. The identity of the other hijackers still at large is a mystery, an intelligencesource in the US said.

"In some cases they don't even have any names. In other cases they are not sure of the right spellings for the Arabic," the source said. "The search through the INS (Immigration and Naturalisation Service) records is turning out to be a nightmarish experience. It goes back months or years."

The FBI on Thursday warned of fresh attacks in the next "several days" but gave no details about targets.